The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with the removal of DC artifacts from two and three dimensional magnetic resonance images.
Heretofore, an analog magnetic resonance signal was digitized by an analog-to-digital converter. The digitized data was Fourier transformed and subject to other data processing to generate a resultant image. Commonly, the digitized data was the sum of the magnetic resonance signal and a DC offset. When the DC offset was Fourier transformed and processed, it commonly manifested itself as a bright point or spot in the resultant image. Some processing techniques such as zero filling prior to performing the Fourier transform tended to smear the bright spot. Various techniques have been developed to reduce the deleterious effects of DC artifacts.
In one technique illustrated in U.S. Pat. No. 4,612,504, the DC artifact was shifted to the edge of the resultant image. More specifically, the phase of the RF excitation pulse was altered by 180 between sequential phase encoding views. The DC offset is independent of the RF phase change and has a limited bandwidth. Accordingly, the artifact was reduced to small regions at the edge of the field of view and did not interfere with the central region of the image. A primary disadvantage of this technique was that the artifact was not removed, only relocated. When the object of interest was not in the center of the field of view, the DC artifact may lie on part of the object.
In another technique, each view was collected twice in first and second almost identical magnetic resonance excitations. However, the RF pulse in one was 180.degree. out of phase relative to the other. The two common views were then subtracted such that the magnetic resonance component sums and the DC component subtracts. Although this technique successfully removed the DC artifact, it required a doubling of the imaging sequence.
In yet another technique, the DC offset in each view was estimated based on a small number of points at the very beginning and end of the data sampling. It was assumed that at the ends of the sampling period there was no magnetic resonance signal - just noise and DC offset. A small number of the first and last sampled points for both the real and imaginary channels of each view were sampled. The average of these points was assumed to be the DC offset and subtracted from each point in the corresponding view. A primary drawback to this technique was that it did not calculate the DC offset accurately and consistently. Whether the end points of the sample contained magnetic resonance signals and how much varied from view to view, with the imaged object, with scan parameters, and the like. Thus, some views were corrected relatively accurately, while other views had a grossly exaggerated DC offset subtracted. Commonly, this produced a DC artifact in the form of a line down the center of the image.
To eliminate the magnetic resonance component when sampling the DC offset, U.S. Pat. No. 4,616,183, performed a data acquisition with no RF pulse. Either prior to or after the image data acquisition, or both, a similar data acquisition was made using similar or identical scan parameters. This additional data acquisition could be done for each slice, for each echo, or for each transmit/receive RF pulse. The data sampled in the absence of the RF pulse was sampled and averaged to generate real channel and imaginary channel DC offsets for each slice, echo, and transmit/receive pulse. During the reconstruction process, the appropriate DC offset value was subtracted from each view of data. However, this technique failed to produce artifact free images. Although the DC offset compensation sampling was free of magnetic resonance signals, the level of noise was relatively high. In a normal sampling interval, there were not enough samples available to average out and accurately estimate the DC offset. Due to fluctuations in the noise from view to view, the views were corrected with varying degrees of accuracy. Moreover, for scans with long repeat times and a large number of slices and echoes, this technique significantly lengthened the imaging time.
The DC offset artifact has also been removed subsequent to reconstructing the image representation. When the image was reconstructed without filtering and without interpolation, i.e. without a zero filled transformation or post transformation interpolation, the DC artifact was a single pixel of very high intensity. The DC artifact pixel value was zeroed and replaced with the average of its neighbors. One of the drawbacks of this technique was that although the artifact was removed, it was not corrected. Further, when filtering or using zero filled transformations, the artifact was spread out in either the phase encoding direction, the frequency encoding direction, or both, over several pixels, particularly in three dimensional scans. Replacing a plurality or line of pixels with the average of adjoining pixel values could create a different but perceptible artifact.
The present invention contemplates a new and improved technique which accurately removes DC artifacts.